The present invention relates to an angular velocity sensor known as a gyroscopic instrument and more particularly, to a high-performance angular velocity sensor having a tuning-fork construction where two vibrator units containing piezoelectric elements are coupled to each other and a method of fabricating the same.
A conventional gyroscopic inertia navigation system includes mechanical rotor gyros for determining the direction of a moving object, e.g. an airplane or ship.
Such a mechanical gyroscopic system is steady in the performance but bulky in the size thus increasing the cost of production and hardly permitting the application to any small-sized pertinent apparatus.
Also, an oscillator-type angular velocity sensor is known for detecting a "Coriolis force" with its detector while it is vibrating but not rotating. Such a sensor commonly employs a piezoelectric or electromagnetic oscillation mechanism.
The detection of an angular velocity in the sensor is implemented by sensing a vibration torque of a frequency equal to that of the mass of a gyro which is not rotating but vibrating at a constant rate. The vibration torque is known as the Coriolis force generated when an angular velocity is involved.
The oscillator-type angular velocity sensor can detect the amplitude of the vibration torque to determine an angular velocity. Particularly, a variety of the oscillator-type angular velocity sensors employing piezoelectric elements have been introduced (for example, as depicted in the Proceeding of Japanese Institute of Aviation and Space, Vol.23, No.257, pp. 339-350).
One of the conventional tuning-fork structure oscillator-type angular velocity sensors will now be described referring to FIGS. 12 to 14. The arrangement of the angular velocity sensor is best illustrated in FIG. 12 which consists mainly of four piezoelectric bimorphous elements serving as an actuator, a monitor, and a first and a second detectors. The actuator 101 is orthogonally coupled by a joiner 105 to the first detector 103 constituting a first vibrator 109 while the monitor 102 is orthogonally coupled by another joiner 106 to the second detector 104 constituting a second vibrator 110. The first 109 and the second vibrators 110 are coupled to each other by a connector 107 which is supported at a point by a support 108, thus constructing a tuning-fork structure.
When the actuator 101 of piezoelectric bimorphous element is loaded with a sine-wave voltage signal, its inverse piezoelectric effect causes the first vibrator 109 to vibrate. Then, the action of the tuning-fork structure results in vibration of the second vibrator 110.
Accordingly, the monitor 102 of piezoelectric bimorphous element generates a charge on its surface through its piezoelectric action. The charge is proportional to the sine-wave voltage signal applied to the actuator 101. Hence, a constant, continuous action of vibration is developed by controlling the sine-wave voltage signal to the actuator 101 so that the charge generated by the monitor 102 remains uniform in the amplitude.
The action of the angular velocity sensor for producing an output corresponding to an angular velocity involved will be explained referring to FIGS. 13 and 14. FIG. 13 is a top view of the angular velocity sensor of FIG. 12. As shown, the turning movement at an angular velocity of .omega. produces a Coriolis force on the first detector 103 which vibrates at a speed of v. The Coriolis force is at a right angle to the speed v and its magnitude is 2 mv.omega. (where m is the equivalent mass at the distal end of the first detector 103).
As the first detector 103 vibrates at the speed v, the second detector 104 is responsive to vibrate at -v and a Coriolis force on the second detector 104 is -2 mv.omega.. The two detectors 103 and 104 are stressed in opposite directions by their respective Coriolis forces thus producing charges on the surface through their piezoelectric actions.
When the speed v of vibration created by fork oscillation is expressed by: EQU v=a.multidot.sin .omega..phi.t
where a is the amplitude of the vibration and .omega..phi. is the period of the vibration, the Coriolis force is: EQU F.sub.C =a.multidot..omega..multidot.sin .omega..phi.t
While the angular velocity .omega. is proportional to the vibration amplitude a, the Coriolis force causes either of the two detectors 103 and 104 to deflect in one direction. Hence, the surface charge Q on the detectors 103 and 104 is expressed by: EQU Q.varies.a.multidot..omega..multidot.sin .omega..phi.t
When the vibration amplitude a is controlled to a constant, EQU Q.varies..omega..multidot.sin .omega..phi.t
As understood, the surface charge Q is found proportional to the angular velocity .omega. and can be converted to a direct current signal through synchronous transaction at .omega..phi.t.
In theory, if the angular velocity sensor is subjected to a translational movement rather than the rotation, its two detectors 103 and 104 produce two charges of the same polarity and their resultant DC signals are offset each other generating no output.
However, the two signals derived from the unwanted charges are not always canceled to zero because of a symmetrical error and a difference in weight between the two, left and right, prongs of the tuning-fork structure of the conventional angular velocity sensor in which a plurality of piezoelectric bimorphous elements are assembled in a relatively complex manner and may not be identical in the quality.
For overcoming the disadvantages, best care is taken to assemble the tuning-fork structure to ensure the symmetry and balance of the fork structure. So far, such efforts are found unsuccessful and fail to cancel both undesired signals. The two unwanted signals cause the sensor to deteriorate the thermal characteristics and become oversensitive to external interruption or vibration.
The piezoelectric element essentially includes two electrodes at both sides regardless of bimorphous or unimorphous type. The electrodes are commonly formed of silver materials printed onto both surfaces of an piezoelectric substance and thus, become rarely identical in weight, size, and quality due to the presence of unavoidable burrs impairing the symmetry of the tuning-fork structure.